1. Field of the Invention
The present invention relates to a filter unit using a detachable roll filter and a projection-type display apparatus using the filter unit.
2. Description of Related Art
Projection-type display apparatuses such as a projector have been used in a variety of circumstances, from home-theater to commercial uses. Projection-type display apparatuses include image display elements, such as a liquid crystal panel and a DMD (digital mirror device), and magnify and project an optical image formed by concentrating strong light from a light source, such as a lamp, onto the image display elements and modulating the strong light in response to an input image signal. The image display elements subjected to strong light, the light source itself the apparatus power supply, etc. may be thermally damaged unless they are cooled properly. Therefore, cooling fans for obtaining a strong cooling effect generally are used in projection-type display apparatuses so as to blow air into the apparatuses from the outside.
However, when a flow of air is produced by a cooling fan blowing in air, dust in the air enters the apparatus, so that it is necessary to remove the dust by providing the air intake with a filter. However, when the dust accumulates on the filter, the cooling effect drops. Therefore, many projection-type display apparatuses use detachable roll filters.
When a detachable roll filter is used, the cooling effect can be kept constant by taking up the filter when a predetermined amount of dust accumulates on the filter. However, since the length of the filter that can be taken up is fixed, the filter needs replacing when it is taken up entirely. To know an indication of the timing to replace the filter, it is necessary to detect the used amount or remaining amount of the filter.
For example, JP 2008-309913 A discloses a filter device configured to be capable of detecting a transferred amount of a filter when the filter is taken up. FIG. 8 is a front view showing the basic configuration of the filter device. FIG. 9 is a perspective view of a gear base of the filter device. Note that some parts are omitted and not shown in FIGS. 8 and 9.
The filter device is composed of a filter unit 60 and the gear base 61. The filter unit 60 includes a filter holder 62, an opening 63, an opening wall 64, an airflow aperture 65 and a take-up shaft side coupling unit 66. The filter unit 60 is equipped with a filter cartridge 67. The gear base 61 includes a gear motor 68, a driving shaft side coupling unit 69, magnets 70 and a magnetic sensor 71. The magnets 70 are placed at predetermined positions on the periphery side of the driving shaft side coupling unit 69. The magnetic sensor 71 detects a change in magnetic force brought by rotation of the magnets 70.
In this configuration, the rotation of the driving shaft side coupling unit 69 is detected with the magnets 70 and the magnetic sensor 71, and based on the amount of rotation corresponding to a detection signal of the magnetic sensor 70, the amount of the filter transferred when the filter being taken up is detected. By summing the transferred amounts of the filer detected with the take-up of the filter, the currently used amount of the filter is calculated.
However, there is normally a small difference between the transferred amount of the filter detected by the magnetic sensor 17 and the actual transferred amount of the filter. For example, there is some slack in the filter in the filter cartridge 67 in a rolled state. Thus, a small difference develops between the amount of rotation of the driving shaft side coupling unit 69 transmitted through the take-up shaft side coupling unit 66 and the transferred amount of the filter as the filter is taken up. Since the detection output of the magnetic sensor 71 is proportional to the amount of rotation of the driving shaft side coupling unit 69, the transferred amount of the filter detected by the magnetic sensor 71 and the actual transferred amount of the filter do not correspond with each other precisely. Such a difference is amplified when summing the transferred amounts of the filter.
Further, JP 2008-309913 A does not describe an optimum way to correlate the detection result provided by the magnetic sensor 71 and the used amount of the filter precisely.